The pineal gland is a tiny organ in the center of the brain that played
an important role in Descartes' philosophy. He regarded it as the principal
seat of the soul and the place in which all our thoughts are formed.
Today, René Descartes (1596–1650) is mainly known because of his
contributions to mathematics and philosophy. But he was highly interested in anatomy
and physiology as well. He paid so much attention to these subjects that it has
been suggested that “if Descartes were alive today, he would be in charge of
the CAT and PET scan machines in a major research hospital” (Watson 2002, p.
15). Descartes discussed the pineal gland both in his first book, the Treatise
of man (written before 1637, but only published posthumously, first in an
imperfect Latin translation in 1662, and then in the original French in 1664),
in a number of letters written in 1640–41, and in his last book, The passions
of the soul (1649).
The pineal gland played an important role in Descartes' account because
it was involved in sensation, imagination, memory and the causation of bodily
movements. Unfortunately, however, some of Descartes' basic anatomical and
physiological assumptions were totally mistaken, not only by our standards, but
also in light of what was already known in his time
In Descartes' description of the role of the pineal gland, the pattern
in which the animal spirits flow from the pineal gland was the crucial notion.
He explained perception as follows. The nerves are hollow tubes filled with
animal spirits. They also contain certain small fibers or threads which stretch
from one end to the other. These fibers connect the sense organs with certain
small valves in the walls of the ventricles of the brain. When the sensory
organs are stimulated, parts of them are set in motion. These parts then begin
to pull on the small fibers in the nerves, with the result that the valves with
which these fibers are connected are pulled open, some of the animal spirits in
the pressurized ventricles of the brain escape, and (because nature abhors a
vacuum) a low-pressure image of the sensory stimulus appears on the surface of
the pineal gland. It is this image which then “causes sensory perception” of
whiteness, tickling, pain, and so on. “It is not [the figures] imprinted on the
external sense organs, or on the internal surface of the brain, which should be
taken to be ideas—but only those which are traced in the spirits on the surface
of the gland H (where the seat of the imagination and the ‘common’ sense is
located). That is to say, it is only the latter figures which should be taken
to be the forms or images which the rational soul united to this machine will
consider directly when it imagines some object or perceives it by the senses”
(AT XI:176, CSM I:106). It is to be noted that the reference to the rational
soul is a bit premature at this stage of Descartes' story because he had
announced that he would, to begin with, discuss only the functions of bodies
without a soul.
Imagination arises in the same way as perception, except that it is not
caused by external objects. Continuing the just-quoted passage, Descartes
wrote: “And note that I say ‘imagines or perceives by the senses’. For I wish
to apply the term ‘idea’ generally to all the impressions which the spirits can
receive as they leave gland H. These are to be attributed to the ‘common’ sense
when they depend on the presence of objects; but they may also proceed from
many other causes (as I shall explain later), and they should then be
attributed to the imagination” (AT XI:177, CSM I:106). Descartes' materialistic
interpretation of the term ‘idea’ in this context is striking. But this is not
the only sense in which he used this term: when he was talking about real men
instead of mechanical models of their bodies, he also referred to ‘ideas of the
pure mind’ which do not involve the ‘corporeal imagination’.
Descartes' mechanical explanation of memory was as follows. The pores or
gaps lying between the tiny fibers of the substance of the brain may become
wider as a result of the flow of animal spirits through them. This changes the
pattern in which the spirits will later flow through the brain and in this way
figures may be “preserved in such a way that the ideas which were previously on
the gland can be formed again long afterwards without requiring the presence of
the objects to which they correspond. And this is what memory consists in”
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